Vertical multiple passage drainable heated surfaces with headers-equalizers and forced circulation

ABSTRACT

The present invention discloses improved heated surfaces (HS) with vertical multiple passage panels or vertical serpentine coils from straight tubes with connections between them by top and bottom bends. In HS with tube bends there is not any mixing headers—each circuit has a single tube from inlet header to outlet header. This increases mass velocity of flow and improves stability and temperature regulation of tubes. The bottom bends have holes. The bottom bend holes of the adjacent passes are connected with drain header by drain stubs. Each header serves to drain the adjacent tube passes and as equalizing header of pressure/flow. It will help to decrease multivaluedness and maldistribution of flow between parallel tubes of the module. Such design of HS noticeably decreases the corrosion of tubes.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. provisional application No. 62/062,055 filed on Oct. 9, 2014.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is the heated surfaces (HS) for application in the different technical fields—power industry, metallurgy, chemical industry, etc. Below the invention is considered in the context of power industry.

2. Background Art

For units of power industry which are based on Brayton and Rankine cycles the general tendency is increasing, the initial parameters—pressure and temperature to improve, the thermal efficiency (TE) and decrease carbon dioxide emissions. In the modern conventional units the pressure is well above the critical pressure. The boilers of such units are once through type.

The same tendency with pressure increasing can be seen in the case of Combine Cycle Power Plants (CCPP) as well. At present time the pressure in steam generators of CCPP is below of critical pressure. However in the future heat recovery steam generators (HRSG) the pressure will be above critical pressure. The cycle of CCPP with HRSG of supercritical pressure (SCP) can have the thermal efficiency well above 60%. The SCP HRSG could be realized as once through boilers (OTHB) only. The subcritical conventional boilers and HRSGs can be as once through (circulation ratio is equal one) units and with forced circulation (circulation ratio is above one) as well. These two types of units are the subject of invention. The conventional boilers and the HRSGs of subcritical pressure with natural circulation are not considered here based on essence of invention.

The main problems of once through units and units with forced circulation are corrosion/erosion, temperature regime of heated surfaces, and hydrodynamic instability. In accordance with Electrical Power Research Institute data the main failures of boilers and HRSGs are as result of corrosion of heated surfaces. The suggested invention can improve the effectiveness and reliability of multiple passage heated surfaces and increase TE of whole units.

SUMMARY OF THE INVENTION

A principal peculiarity of suggested heated surfaces with forced circulation are the vertical multiple passage panels (usually used in conventional boilers) and vertical serpentine coils (usually used in HRSGs) from some rows of straight tubes with connections between them by top and bottom bends. Such types of HS can be used for different elements of boilers and HRSGs—water preheaters (WPHTR), economizers (EC), evaporators (EV), superheaters (SH), and reheaters (RH).

In HS with tube bends there are not any mixing headers—each circuit goes like a single tube from inlet header to outlet header. It allows increasing mass velocity of steam/water or steam-water mixture for subcritical pressure (or supercritical fluid) and improves stability of flow and temperature regime of tubes.

To have opportunity for water drain at the lower point of bottom bends there are the holes. The bend holes of the adjacent passes are connected with drain header by pipe stubs. Each header serves for drain of the adjacent tube passes. At the same time the header can serve as equalizing header of pressure/flow. In the case of the different thermal-hydraulic characteristics of parallel tubes there is opportunity for bypass flows. It will help to decrease multivaluedness and maldistribution of flow between parallel tubes of the module. Besides such design of HS noticeably decreases the corrosion when the units are not in operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be discussed in further detail below with reference to the accompanying figures in which:

FIG. 1 is a scheme of Benson HRSG with once through evaporator;

FIG. 2 is a scheme of conventional boiler with N-shaped evaporator panel and forced circulation;

FIGS. 3a-3h are multiple passage panels of conventional once through boilers;

FIG. 4 is a typical vertical serpentine drainable coil with bottom header and forced circulation;

FIG. 5 is a vertical serpentine coil of HRSG with drainable header-equalizer;

FIG. 6 shows varying drain stub and drain pipe diameters for a vertical serpentine coil of HRSG with drainable header-equalizer;

FIG. 7 is a multiple passage panel of convention once through boiler with drainable header equalizer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

All types of thermal power plants are emitting a lot of waste heat into the atmosphere. To improve the thermal efficiency and decrease carbon dioxide emissions a noticeable portion of waste heat can be captured and used for the different goals. Let's consider the issue in the context of Combine Cycle Power Plants (CCPP) as one of the most effective cycles in power industry. In this case the heat of exhaust gas after gas turbines (GT) can be used in heat recovery steam generators. The steam after HRSG goes into steam turbine (ST) which drives electrical generator. The thermal efficiency of such combined cycle (Brayton and Rankine cycles) can be above 60%. In contrast a single ST cycle of subcritical pressure is limited around 40%, the best modern GTs have thermal efficiency about 45%. The modern units with ST cycle of supercritical pressure have achieved 43-44%. In this respect the future tendency in this branch of power industry should be the CCPP with HRSG of supercritical pressure (SCP). The SCP HRSG could be realized as once through boilers (OTHB) only (it means a forced circulation in all elements of boiler including zone of maximum thermal capacity). At the same time the conventional boilers of supercritical pressures are in operation during many years already.

There are some publications regarding SCP HRSG with horizontal design of heated surfaces and vertical gas flow. However there is no information about any practical application of this approach. The most common technology of subcritical horizontal OTHB is Benson HRSG licensed by Siemens (FIG. 1, prior art). FIG. 1 shows the Benson HRSG with feed water inlet 104, star distributor 105, downcamera 106, a first evaporator 103, a second evaporator 102, separator 107, and super heater 101. Unfortunately Benson HRSG has some serious problems with evaporator (EV) design—the very small velocities of flow in cold section and EV instability; a very complex design of hot EV section with not good temperature regime of tubes; two-phase flow distribution by star-distributor on the hot EV section inlet is very complex and not reliable, etc.

One of the most important and key element in once-through boilers and HRSGs is evaporator (pseudo EV in the case of SCP). It is a reason for consideration below of HS as the EVs in the first turn. In subcritical EV there is two-phase flow (steam/water mixture) in all range of operational parameters. In supercritical boilers two-phase flow could be under partial loads. Two-phase flows have very complex structure, hydrodynamics, and very complex behavior of heat transfer and hydraulic resistance coefficients. The stratification of two-phase flows, deposition of salts, corrosion/erosion issues, critical heat fluxes, and instability are the major concerns for designers of once-through HRSGs and conventional boilers.

There is a good experience in design and operation of conventional once through units and units with forced circulation (FIG. 2, prior art) with different levels of pressure. The working fluid (water for subcritical pressure or supercritical fluid for supercritical pressure) from inlet manifold goes throw high pressure (HP) economizer to HP evaporator (for subcritical pressure) or pseudo evaporator PEV (for supercritical pressure). FIG. 2 shows a scheme of a conventional boiler with N-shaped evaporator panel and forced circulation having a feed water pump 201, economizer 202, evaporator 203, circulation pump 204, second stage evaporator 205, super heater 206, outlet for super heater 207, separator 208, and valve 209. As a result of heat absorption from hot exhaust gas an enthalpy of working fluid will change from water inlet value up to value of slightly superheated steam in EV outlet and strong superheated steam in SH coils. As rule, the modern units have a reheat system as well.

The tube panels of conventional boilers with forced circulation (EC, EV, SH and RH) have the very different layouts. For example, on FIG. 2 the EV panel was implemented as N-shaped panel with vertical tubes. Different manufactures are used the different types of tube panels (FIG. 3, prior art). FIGS. 3a-3h show multiple passage panels of conventional once through steam generators. FIG. 3a show a N-shaped panel with vertical tubes. FIG. 3b shows a modified N-shaped panel. FIG. 3c shows a standard panel with horizontal tubes. FIG. 3d shows a modified panel with horizontal tubes. FIG. 3e shows a vertical multiple passage panel. FIG. 3f shows a modified vertical multiple passage panel. FIG. 3g shows vertical panels with even number of passages. FIG. 3h shows vertical serpentine panels. Each type of panels has the advantages and disadvantages. Many problems were resolved regarding heat transfer and hydrodynamics of flows in such panels. One of the main problems of multiple passage panels with vertical tubes (types—a, b, e, f, g, and h) is corrosion of internal surface of tubes. As it can be seen from FIG. 3 in these panels there are some non-drainable passages. During the shutdown period some water can accumulate in the bottom bends of these passages that result in strong corrosion.

In the coils of HRSGs there is similar problem. For example, in the earliest types of serpentine coils there were some non-drainable bottom bends. Because of strong corrosion the bottom bends were replaced with bottom headers (FIG. 4, prior art). It helps to decrease a rate of corrosion. However the hydrodynamics of such vertical serpentine drainable coils is complex enough. Besides such type of coils can't be used for EVs because of possibility for maldistribution of two-phase flows between parallel tubes.

The tubes of HRSG heated surfaces are the finned tubes usually. In the case of relatively big heat fluxes some rows of HRSG can be manufactured from the bare tubes. A principal peculiarity of suggested heated surfaces for HRSG conditions is the vertical serpentine coil from some rows of straight tubes with connections between them by top and bottom U-bends 506 (FIG. 5). A direction of water flow in the coil could be counter flow or parallel flow with exhaust gas flow. A tube (finned or bare) layout could be a staggered or an inline.

FIG. 5 shows gas baffle keeper 515 connected to gas baffle 516. An inlet header 502 and outlet header 501 are contained within gas baffle 516. Inlet header 502 and outlet header 501 are connected by evaporator coil 503. The evaporator coil has bottom U-bends 506. The bottom U-bends 506 are connected to drain stubs 507 which are connected to elemental header-equalizer 509 which are connected to drain bypass 508 which are then connected to bellows 513. The bottom U-bends 506, drain stubs 507, elemental header-equalizers 519, drain bypasses 508, integral header-equalizers 509, water cooled wall inlet header 510, water cooled wall outlet header 511, and water cooled wall tubes 504 are all contained within drain box 517. Water cooled wall inlet header 510 and water cooled wall outlet header 511 are connected by water cooled wall tubes 504. Drain box 517 has water cooled wall 505 (which is adjacent to water cooled wall tubes 504) which is connected to a gas baffle keeper 510 sitting on top of liner 518. Liner 518 sits on top of casing 514. The drain bypasses 508 pass through the liner 518 and casing 514 into the bellows 513.

Let's consider the specific of suggested HS for EV of HRSG because the physics of two-phase flow is more complex as the single flows. In case of EV with tube bends (FIG. 5) there are not any mixing headers—each circuit goes like a single tube from inlet header to outlet header. It allows increasing mass velocity of steam-water mixture (or supercritical liquid) and improves stability of flow and temperature regime of tubes. To have opportunity for water drain at the lower point of bottom bends there are the holes. The bend holes of the adjacent rows are connected with drain header by pipe stubs. The pipe stub diameter is noticeably less than EV tube diameter. It means that water flow goes mainly through the EV tubes. Each header serves for drain of two adjacent tube rows. The sizes of such headers should be smaller than regular bottom headers of standard design.

At the same time the header can serve as equalizing header of pressure/flow. In the case of the different thermal-hydraulic characteristic of parallel tubes there is opportunity for bypass flows. It will help to decrease multivaluedness and maldistribution of flow between parallel tubes of the module. There is sense to underscore that bottom portion of coil (from the hole in bottom bend to drain pipes) will be filled in with water (for subcritical EV) or heavy phase (for supercritical EV). It is a result that in bottom bends there is a centrifugal force, which in many times more than force of gravitation. Besides in the bottom bend the both vectors of centrifugal force and force of gravitation are coincided (both are acting in downward direction). Both forces are proportional to density of medium. It means that integral force for water will be more than for steam (for subcritical pressure) or for heavy phase will be more than for light phase (for supercritical pressure). Besides, in vertical downward two-phase flows, as rule, velocity of droplet is more than velocity of steam. It means that centrifugal force for droplet in bottom bend will be more than for steam. This results in that all operation conditions the bottom portion of coil (from drain stubs and below) will be filled in with water (for subcritical EV) or heavy phase (for supercritical EV). This is very important for thermal-hydraulic processes in the coil.

In some HRSG design conditions drain system is situated in the area of relatively hot exhaust gas. To avoid thermal-mechanical problems the drain system should have the proper temperature regime. It can be achieved by circulation of small amount of water (or supercritical liquid) through drain system. For this goal two adjacent drain lines are connected by drain cross over (drain bypass) pipe (FIG. 5). The diameter of drain cross over pipe is noticeably less than EV tube diameter and should be calculated in such a way to guarantee the proper temperature regime of the drain system. To decrease the number of drain pipe penetrations through the casing the design of drain assembly could be as it is shown on FIG. 5. All pipe penetrations are realized with help of bellows. Of course for sizing of cross over pipes (drain bypass pipes) it is needed to take into account the difference in pressure drop between the proper headers. On FIG. 5 the drain headers are depicted for cold conditions (unit is not in operation). In hot conditions (unit in operation) the headers will move down to bottom liner as result of coil expansion with temperature. In the case the temperature regime of drain system will be normal even with relatively small water flow in drain bypass lines.

Temperature regime of drain system can be reliable under small water bypass if it is situated out of the main exhaust gas flow (in the area of relatively stagnant gas flow). Such scenario can be realized with help of gas baffle plates (FIG. 5). Upper portion of plates should be fixed on the EV (EC, SH, etc.) tubes above bottom bends. Lower part should be situated in the baffle keeper. On this figure EV (EC, SH, etc.) coil is fixed on the top of HRSG (at some designs a coil can be fixed in the bottom). In this case the coil will expand in downward direction. A height of the sealing assembly (baffle keeper) should guarantee the coil expansion in all operational conditions. The upper portion of baffle plates should have such height to minimize exhaust gas flow through the box with drain system. The drain box includes bottom bends, drain stubs, headers-equalizers, drain pipes and confined by gas baffle plate. The box of drain system has to have the gas baffle plates on all four sides. In the case of multi wide HRSG the gas baffle plates are installed between the modules as well to decrease the gas bypass.

In the case, when the coil is situated in very high range of gas temperatures, the temperature regime of drain system can be kept on reliable level with help of water cooled walls. The walls could be fabricated from membrane tubes to minimize gas bypass. Water for the wall is used after EC before going to EV. It is possible as well to take water for the wall between the sections of EC. Thermodynamic efficiency is taken into account in each case. A direction of water flow in the wall could be counter flow, parallel flow, or perpendicular with exhaust gas flow. Designer has to take into account the peculiarities of temperature regimes of the coil and water cooled wall in the contact area of tubes with different wall temperatures. In the case of multi wide HRSG the gas baffle plates should be installed between the water cooled boxes of the different modules. In most cases the water cooled walls will not be necessary.

Temperature regime of once-through HRSG EV tubes could be different from HRSG EV tubes with natural circulation. In OTH HRSG EV subcritical or supercritical pressures could be a zone with deterioration of heat transfer. It means that under any enthalpy of fluid there is a jump in tube wall temperature. The value of temperature jump depends on parameters of exhaust gas flow, as well of water pressure, mass velocity, and heat flux for given geometry of coil tubes. For any combination of these parameters temperature jump could be strong enough. To improve the tube temperature regime the different types of intensificators can be used (rifled tubes, inserts, etc.).

A special attention should be paid to bottom bends and connections with drain stubs (FIG. 5). To avoid excessive stresses the material and geometry of the transition tees should be designed properly. The drain system should be a flexible enough to compensate the possible difference in expansions of the coil and drain system.

The HRSG can be operated under supercritical and subcritical pressures. Under nominal conditions a unit can be supercritical but under part loads the pressure in system can be subcritical. Besides on the EV outlet a two-phase flow could be, but not superheated steam. To prevent a steam-water mixture going in superheater the special separators should be on the outlet of EV (see FIG. 2). At the same time the assembly of the separators and the water tanks can help to manage the proper temperature regime of EV under partial loads of HRSG. A control of water mass velocity and steam quality on the EV outlet can be done by the feed water pump(s) or by the special recirculation pump(s). Exploitation of the feed water pump(s) or the special recirculation pump(s) should be chosen based on technical/economical evaluations of the proper schemes (price of pumps, price of electricity, relative duration of part loads, etc.).

FIG. 5 shows an inlet and outlet header that may be connected to one or more vertical evaporator coils. The evaporator coils have bypass drains connected to the bottom of the evaporator coil. The drain bypasses ultimately lead to a bellows on the other side of the casing. The vertical evaporator coil may further have a water cooled inlet header and water cooled outlet header connected by water cooled wall tubes contained within a water cooled wall and connected to the casing by a gas baffle keeper.

FIG. 6 shows the bottom U-bend 606 of the vertical evaporator coil. The bottom U-bends 606 can be connected by drain stubs 607. The drain stubs 607 do not all need to be the same diameter, the drain stubs 607 and bypass lines 608 can vary in diameter across the vertical evaporator coil. The widest diameter drain stub 607 can be either on the inlet header or outlet header side of the vertical evaporator coil. The drain stub 607 and the bypass line 608 are connected at the elemental header-equalizer 609.

Relatively simpler thermo-mechanical situation is in the header—equalizer and the drain system of multiple passage panels of conventional boilers (FIG. 7). The tubes 719 of the panel are connected by tube stubs 707 with header-equalizer 709. The headers—equalizer are situated outside of casing. The drain pipes 708 are also situated outside of combustion chamber (or gas duct). In this sense the temperature regime of drain system will be reliable because of there is no the additional sources of heat.

Special attention should be paid to the position of header—equalizer. The header should be a horizontal orientation to avoid the effect of possible gravitational component in pressure drop (it is specific of vertical and inclined headers). Header should be situated below the lowest row of the panel. It can guarantee that header will be filled in with water (for subcritical pressure) or heavy fluids (for supercritical pressure). This provision is very important for stability of flow.

The length of heated tubes and pipe stubs of panel different circuits in the connection points with headers could be different. In this respect the special attention should be paid to equal pressure drop. The possible way to do it is to adjust the sizes of the holes in the bottom bends and in the header—equalizers in such manner to have the equal pressure drop in all parallel circuits.

While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, and examples herein. The invention should therefore not be limited by the above described embodiment, and examples, but by all embodiments within the scope and spirit of the invention. 

What is claimed is:
 1. Heated surfaces (HS) with forced circulation of liquids/gases, comprising: a vertical multiple passage panel or serpentine coil comprising rows of vertical straight tubes with connections between them by top and bottom bends; each bottom bend having a drain stub; each drain stub being connected to a header; all headers are located below the bottom bends; each header being connected to at least one bypass line for circulating water; thus providing a stable operation of heat exchange elements of boilers and steam generators; wherein each HS absorbing heat from exhaust gas.
 2. The heated surfaces of claim 1, wherein the drain stub is connected to the header, which is an elemental header; further comprising: at least one integral header receiving the bypass from the elemental header and outputting at least one drain pipe for water draining into a water accumulator.
 3. The heated surfaces of claim 2, further comprising drain stubs entering the integral header.
 4. The heated surfaces of claim 2, wherein the accumulator is located outside a casing; and the drain pipes go through the casing via holes.
 5. The heated surfaces of claim 4, wherein the holes in the casing have larger diameter than the drain pipes allowing thermal expansion of the drain pipe.
 6. The heated surfaces of claim 2, wherein both types of headers—integral and elemental serve as equalizers thus regulating a pressure and a flow liquid/vapor in parallel circuits.
 7. The heated surfaces of claim 2, wherein water always passes through the headers to cool down the bypass lines and the drain pipes.
 8. The heated surfaces of claim 1, wherein a diameter of the stub is smaller than a diameter of the bottom bend.
 9. The heated surfaces of claim 1, wherein a diameter of the stub is at least twice smaller than a diameter of the bottom bend.
 10. The heated surfaces of claim 1, further comprising: a drain box covering the bottom bends, the drain stubs, the drain pipes, the bypass lines and the headers; the box including water cooled wall tubes to keep proper temperature conditions in the drain box.
 11. The heated surfaces of claim 1, wherein each water cooled wall tube is connected to an inlet header and an outlet header.
 12. The heated surfaces of claim 1, wherein a pressure inside the panel or the coil is a subcritical pressure.
 13. The heated surfaces of claim 1, wherein a pressure inside the panel or the coil is a supercritical pressure.
 14. The heated surfaces according to claim 1, where the sizes of holes in the bottom bends and in the headers of multiple passage panels or serpentine coils are adjusted in such manner to have an equal pressure drop in all parallel circuits. 